The deposition of thin, transparent, conductive metal oxide films on glass or quartz substrates has been practiced commercially for some years. In general, commercial processes apply such films by spraying reactive metal compounds, such as tin tetrachloride, onto a hot substrate in the presence of oxygen. However, these films are not conductive enough or uniform enough for certain applications. Thus, other methods of applying the films have also been tried.
Chemical vapor deposition processes are useful, but they require high temperatures and high melting substrates and they are expensive. Further, the films are not as conductive as films applied by other means.
Sputtering processes can apply uniform metal oxide coatings, but such processes are even more expensive and high-quality glasses must be used as substrates.
One application for transparent, conductive metal oxide films on glass which is of increasing importance is their use in liquid crystal cells. These cells basically comprise two metal oxide-coated glass electrodes with a layer of liquid crystal material between them. The metal oxide films must be highly conductive and transparent for good cell contrast. Further, the glass substrates must also be of high quality to prevent impurities such as alkali metal ions from migrating into the liquid crystal material during operation, where they have an adverse effect on the alignment and lifetime of such cells.
Recently, it has been found that inexpensive soda glasses can be treated with a corona discharge to deplete the surface of the glass of alkali metal ions. For applications such as liquid crystal cells, this ion depletion substantially upgrades the surface region of inexpensive glasses and allows one to use inexpensive glass for the cell electrodes, with concomitant economies. The removal of alkali metal ions from the glass surface which is to be coated allows conductive films of improved quality and uniformity to be deposited.
Stockdale et al., in a copending application Ser. No. 371,608, filed June 20, 1973, now abandoned, have disclosed a method whereby a surface of an inexpensive soda-lime-silicate glass is first ion depleted by exposing the glass to a corona discharge and then a conductive metal oxide film is applied by conventional spray techniques. It would be desirable however, to be able to apply improved metal oxide coatings onto the ion depleted glass by a one-step technique providing it would not have an adverse effect on the glass surface to be coated.